This invention relates generally to the field of electrical power conversion, and more particularly to high efficiency rectifier solutions.
A totem pole rectifier is a known bridge-less circuit used to rectify an AC line input voltage. It is understood as a solution with potential for highest achievable efficiency (>99%). Conventional versions of totem-pole rectifier circuits include those shown on FIG. 1 to FIG. 3.
An important feature of the totem pole rectifying circuit is that it is not only a bridge-less (BL) solution, but also possibly a diode-less solution. As such, it can incorporate no forward voltage drop devices, which means it can provide very high efficiency at light load conditions. Another important advantage is its compactness because the number of components is low when compared with most other bridge-less circuits. This feature is even more remarkable if CCM (Continuous Conduction Mode) method would be used to control switches to provide high power handling capability. However, there are several limitations which make this solution relatively difficult to implement, and as a result it has not been widely adopted and used.
A first possible implementation of this topology is represented in FIG. 1. It consists of two rectifying diodes 1 and 2 and a shaping branch with MOSFETs 3 and 4 which are operated so that a current through smoothing inductor 101 is shaped to follow a predefined reference. Because diodes 1 and 2 are forward-drop featured devices, this circuit is not a true diode-less solution and does not provide significant advantages when compared with other BL circuits.
Another implementation is represented on FIG. 2. This true diode-less topology includes two rectifying MOSFETs 5 and 6 and two shaping MOSFETs 7 and 8. While this solution benefits from the diode-less character of the circuit, it does, however, feature a significant disadvantage. Slow body diodes of MOSFETs 7 and 8 significantly deteriorate the potential of the circuit in CCM by excessive reverse recovery current that induces high switching losses and may decrease the MOSFETs reliability when CCM control method is used. This effect can be slightly enhanced by using MOSFETs with fast body diode, but with current state-of-the-art devices this does not achieve a significant improvement in efficiency over other conventional BL solutions.
To overcome this problem, there are three basic possibilities known from the prior art.
A first option is to control the shaping switches 7 and 8 with BCM (Boundary Conduction Mode), e.g., at the border of DCM (Discontinuous Conduction Mode) where current through a smoothing inductor 9 periodically falls to zero at the end of switching period, and at this instant the next switching cycle is initiated. An optimized version of this method implements a valley switching technique and hence further decreases capacitive turn-on losses. This method significantly decreases reverse recovery current of body diodes inherently contained in Silicon MOSFETs 7 and 8.
A second option is an extension of the BCM method described in above, where current through smoothing inductor 9 is forced to go negative to achieve ZVS switching for shaping switches 7 and 8 in the entire range of load 11 and the input AC voltage 21.
A third option is to use IGBTs 10-13 instead of MOSFETs as represented in FIG. 3. This solution provides (and requires) the possibility to equip the switching devices with anti-parallel ultra-fast diodes 14-17 because IGBTs inherently do not contain them, and hence it reduces reverse recovery losses.
Each of the three above-mentioned solutions suffer from other problems, however.
Power Density:
When a converter according to FIG. 2 is controlled at the boundary of DCM (BCM), the power handling capability is lower than its CCM controlled counterparts due to the fact that current in the smoothing choke 9 is periodically forced to fall to zero, and hence a high level of power density is difficult to achieve. Power handling in the case of BCM controlled converters can be increased by adding more phases to obtain an interleaved two or more phase converter. This method increases the power handling but still does not provide a very compact solution. In addition, obtaining the proper phase shift between two or more phases for each instantaneous input AC voltage within a half-period and for each load condition is not a trivial task because of varying switching frequency within the input AC voltage cycle. One possible way to handle this is disclosed in Ziegler et al. “Digital Phase Adjustment for Multiphase Power Converters”, U.S. Patent Application Publication No. 2012/0218792, filed Feb. 10, 2012.
Circulating Energy:
When a converter according to FIG. 2 is controlled at the boundary of DCM (BCM), a circulating energy is present in the circuit due to the oscillation between the choke 9 and output capacitance Coss of the MOSFETs 7 and 8. If CoolMOS devices are used with significant Coss=f(Vds) functional dependency, reverse current flowing through choke 9 required to achieve valley or ZVS switching has a quasi-triangular shape and is characterized with high peak amplitude. This increases conduction losses and decreases efficiency at light loads even though a ZVS technique recycles energy stored in output capacitance Coss of switches 7 and 8. Typical waveforms recorded on the totem pole converter according to FIG. 2 and controlled by BCM are represented on FIG. 4. These waveforms correspond to a valley switching technique during frequency limit mode where negative current is obtained only by natural oscillation between inductor 9 and the Coss capacitance of MOSFETs 7 and 8. The amplitude of the oscillation current is about 1 A. Taking into account a targeted 99% efficiency, the power loss associated with this negative current and related circulating energy is high.
Total Harmonic Distortion (THD):
Another consequence of the larger negative current peak flowing through choke 9 that exists during valley switching, and is even larger in the case full ZVS behavior is targeted, is significant deterioration of input current THD when a standard constant on-time method is used within BCM control method. Typical waveforms recorded on the totem pole converter according to FIG. 2 and controlled by BCM are represented on FIG. 5. Negative peak current flowing through choke 9 is increased when the input AC voltage 10 approaches zero. This is directly given by a small positive di/dt and a large negative di/dt of the input current because the slope of current rise/fall is given by a voltage applied on the smoothing choke. In general, to enhance THD, the on-time of the shaping switches 7 and 8 must be varied as a function of the input AC voltage 10 and a load 11, as modeled on FIG. 6.
This functional dependency is given purely by parasitic circuit elements, namely Coss nonlinearity of shaping FETs 7 and 8 and the inductance of the choke 9. It is derived analytically by solving the non-linear system of parametric differential equations given by the strong non-linearity Coss=Coss(Vds) where the parameters are immediate input AC voltage 10 and the load 11. An alternative method based on HW interaction between a control machine and a power stage is disclosed in Ziegler et al. “Input Current Shaping for Transition and Discontinuous Mode Power Converter”, U.S. Patent Application Publication No. 2012/0212276, filed Feb. 10, 2012, describing a method and apparatus to handle deteriorated THD. The method is normally required to be running on a FPGA or ASIC to process the algorithm for the proper on-time generation.
Driving Power:
Driving energy consumption is also not negligible since two shaping MOSFETs need to be driven. Taking into account a targeted >99% efficiency, this part of permanent losses plays an important role in the loss budget and consequently shifts the efficiency curve down.
IGBT:
A circuit according to FIG. 3 offers in contrast high power handling due to CCM capability. However, the bipolar character of IGBT Ic vs. Vice output characteristics with typical forward-drop voltage and significant dynamic “tail current” losses cannot offer better efficiency as compared with a ZVS/valley controlled converter according to FIG. 2. This is especially valid for converters with 400V class DC bus voltage where 600V devices are normally considered.
Electromagnetic Compatibility (EMC):
The rectifying switches 5-6 are commutated each time the zero crossing of the input AC voltage 10 is detected. Typically, the DC bus rails have a parasitic or intended (by means of Y2 capacitors) capacitance 18 referred to a common ground 19. In this case, the commutation of the rectifying switches 5 and 6 causes a common mode (CM) voltage with an amplitude of DC bus voltage (present on capacitor 20) seen on input terminals 21. This rectangular CM voltage features low fundamental frequency equal to frequency of the input AC voltage 10, high amplitude and wide frequency spectrum. Because of that, total EM behavior of the converter is deteriorated.
What is need then are a method and apparatus to effectively overcome above mentioned disadvantages.